This invention relates to perforating systems in general and, more particularly, to high density well perforating systems used in relatively long intervals of a well to be perforated.
It has become fairly common practice in recent years to utilize relatively high density, or a large number of perforating shots per feet, perforating systems in oil well completions. As many as sixteen shots per foot of well interval with a 90.degree. or 120.degree. phasing between perforating shots in the same plane horizontally are typically utilized in such techniques. As many as several hundred feet of potential producing formation may be perforated by high density perforating guns in this manner. Such high density guns may be run by wireline into the cased well bore or in more recent years, such perforating systems may be run in on a production tubing string into the well bore.
In the case of the tubing conveyed perforating systems, a rather lengthy series of sequentially connected perforating guns are run into a cased well bore on the end of a tubing string below a packer. When the sequentially connected string of perforating guns have been run in to the desired depth, the packer is set, and a valve above the packer is used to control the fluid pressure in the tubing string. A reduced pressure (below formation pressure) is usually present in the tubing string and in the wellbore annulus beneath the packer. When this is accomplished the long string of perforating guns connected in series below the packer are sequentially fired (substantially simultaneously) and, since the pressure in the wellbore below the packer is reduced by the control of the fluid in the tubing string, a large pressure differential can exist between the inside of the perforated wellbore casing and the earth formations surrounding the borehole. Rapid out-flow of fluid from the formation into the borehole then backwashes and cleans out the perforations which are accomplished in the conventional manner by shaped charges.
In some tubing conveyed perforating systems. A long series string of perforating guns, up to 1000 feet in length or even longer, may be simultaneously fired. Such guns are typically fired by dropping a sinker bar down the tubing which contains a firing mechanism which detonates a primer cord explosive which in turn detonates the shaped charges in the long string of perforating guns. Detonation is usually accomplished from the top gun of the series string of perforating guns down to the bottom gun and, takes place nearly simultaneously along the entire length of the series connected gun string.
Occasionally, however, a faulty primer cord, or faulty detonation of any single gun in the series of perforating guns run in below the packer on the tubing string, can prevent the guns located beneath it from firing. As each gun is connected sequentially or in series with the guns above it, in such a case a very dangerous situation can exist. High explosives may be present which are still live but unfired in the wellbore without the knowledge of the perforating system operator.
Such live explosives could remain in place for a long time until a well workover is accomplished and the tubing string removed. This could create a potential hazard if unfired high explosives were unknowingly pulled to the surface with the tubing string.
It should be mentioned that in tubing conveyed perforating systems the string of perforating guns may be left in the wellbore on the end of the tubing string after the perforations have been accomplished by firing the guns. Production into the tubing string is accomplished either through or around the perforating gun structure. In other systems, after the string of perforating guns located beneath the packer fired, a mechanism is used to release the fired perforating gun string and drop it to the bottom of the well borehole. In any system it is seen that a dangerous condition could exist if unfired high explosives were still extant in the lower portion of any such tubing conveyed perforating gun and could possibly be detonated when the gun was released and dropped to the bottom of the well borehole, or removed from wellbore.
In short, it is seen that unless some means is provided for detecting whether the entire string of perforating guns beneath the packer has been successfully fired, a dangerous situation could exist in the well borehole which could go undetected and unsuspected.
One of the features of the present invention is to provide a system for detecting whether an entire string of perforating guns which are fired serially, either by wireline, or by a tubing conveyed perforating system, have each been fired by the firing mechanism.
Another aspect of the invention is to provide a method for detection of complete firing of a series sequential string of perforating guns in a well borehole.